A good relationship is hard to come by, but falling for someone a friend once loved, liked, or entertained can cause a riff in the best of friendships. As friends, we'd like to think we'd rise above it — say something like "If he has to date someone, I'm glad it's you" — but emotional virtue is easier in theory than real life.
If we can't curb our emotions then we can at least try to control our behavior, leaving us with two options: keep quiet and hope it blows over, or ask a friend to put your friendship first by refraining from future flirtations. Exes and longtime crushes are obvious contenders, but platonic friends and one-night stands can be just as contentious. If you've ever drawn a line, what guy was it for?